There was a previous thread (now archived) on A.net talking about AA painting the bellies of the 737 and I wanted to share the answer I was able to get. AA Now Painting Bellies?!(by AA737-823 Nov 13 2011 in Civil Aviation)#menu27

Answer from American:

“All 737’s have fluid discharge ports along the bottom Aft fuselage. AA has decided to paint the underbelly of the aircraft for aesthetic purposes and to prevent any contact of these fluids with the bare aluminum skin.”

Looks like they are being painted to slow down corrosion of the bellies. Not sure if they will be painting their older 737s and still no official word if they are looking at a possible new livery.

Nah, I see right through that excuse. I know what they're really doing. Over time, a little paint here, little more paint there, and next thing you know, Bam!, they have an actual paint scheme. It's livery creep, I tell you!

Thanks for the info. I thought it was something like that, but it's nice to see confirmation. I think there are five or six so-painted 738s now...

this seems like a design flaw to me. Why would Boeing put a discharge port any place but at the very rear of the aircraft to prevent fluid from coming in contact with other parts of the fuselage? Is static electricity discharge a concern at that point?

If it is a design flaw, its one that is shared with pretty much all commercial aircraft. Weight and unnecessary complication are two reasons to have the drains where they are. That said, the 737-800/900 does have a problem with the lav blower vent discharging organic matter.

Quoting AirlineReporter (Thread starter):Looks like they are being painted to slow down corrosion of the bellies. Not sure if they will be painting their older 737s and still no official word if they are looking at a possible new livery

What older 737's? Their 737's are fairly recent.
That is true about the venting of fluids on the 737's. There is condensation, you know.
It makes sense to paint underneath to solve that problem. The rest of the plane is still AA.

I was using "older" as a comparison. Yes, all their 737s are relatively new, but the ones they have already taken delivery previously still do not have the painted underside -- and in fact they are older than the ones with the painted bellies.

Quoting Byrdluvs747 (Reply 5):this seems like a design flaw to me. Why would Boeing put a discharge port any place but at the very rear of the aircraft to prevent fluid from coming in contact with other parts of the fuselage? Is static electricity discharge a concern at that point?

Piping it back to tail would add a lot of weight. Quite a bit of fluid can be discharged in flight. Fuel, hydraulic fluid, and grey water all have drain masts. They are on the bottom of the airplane and are designed so that the fluid does not contact the fuselage. Hydraulic fluid (skydrol) will damage paint as well. Rarely does it vent overboard in flight, but sometimes there are reasons for it to. The drain mast for the hydraulic fluid is just aft of the wheel well on the left side of the airplane.

If you have never designed an airplane part before, let the real designers do the work!

Quoting wn700driver (Reply 1):Nah, I see right through that excuse. I know what they're really doing. Over time, a little paint here, little more paint there, and next thing you know, Bam!, they have an actual paint scheme. It's livery creep, I tell you!

Over time, a little paint here, little more paint there, and next thing you know, Bam!

“All 737’s have fluid discharge ports along the bottom Aft fuselage. AA has decided to paint the underbelly of the aircraft for aesthetic purposes and to prevent any contact of these fluids with the bare aluminum skin.”

Looks like they are being painted to slow down corrosion of the bellies. Not sure if they will be painting their older 737s and still no official word if they are looking at a possible new livery.

Yet, CO purposely leave their bellies bare, and they fly a lot more 737s. Like I said on the other thread something isn't right, if AA and CO are doing the exact opposite.

Worked Hard. Flew Right. Farewell, Continental. Thanks for the memories.